Re: Building GTA size map: Possible?
Go into 2D view and zoom out until you see the whole grid, that is how big your map can be. To make something the size of a GTA city you might have to link multiple maps together through level change triggers at the edge of each piece of map. Even if there was a map like that it probably wouldn't be as fun because mutants don't drive cars you can steal and no homeless person in SiN has beaten me up yet.
If you wanted to start on a big project like that you should take some graph paper out and plan the whole city to scale first before going into Hammer.